Technical Field
The invention relates to a resin composition for a wet friction material, a phenolic resin for a wet friction material and a wet friction material.
Related Art
A phenolic resin as a thermosetting resin is widely used mainly as a binder for binding materials that become a substrate of a molded product and is used in various fields since the phenolic resin has excellent mechanical properties, electrical properties and adhesiveness. Particularly, in recent years, the usage of friction materials in which the phenolic resin is used as a binder has been increased in automobiles and railway vehicles.
Among the above, as a friction material which is referred to as a wet paper friction material and used in an automatic transmission of an automatic vehicle or the like, a liquid resol type phenolic resin is generally used. Improvement is increasingly required for the properties of the phenolic resin for a wet paper friction material year by year, and particularly, in order to cope with increase in a load on the wet friction material due to improvement in fuel consumption of a vehicle, improvement in engine output of a vehicle, increase in vehicle weight, and the like, particularly, improvement in adhesion with a substrate and improvement in strength have been increasingly required.
In the general phenolic resin, the molecular weight can be adjusted by mainly setting a reaction molar ratio (F/P) of phenols (P) and aldehydes (F) as materials. As the reaction molar ratio becomes higher, the molecular weight increases and mechanical properties such as strength or the like also increase. However, when the molecular weight is high, the impregnation properties of the substrate with the phenolic resin are deteriorated, and also, the resin becomes too hard and brittle. As a result, a sufficient adhesive force may not be obtained.
In addition, by modifying the phenolic resin using various modifiers, it is possible to impart various functions to the resin. Aralkyl-modified phenolic resins and alkylbenzene-modified phenolic resins having excellent heat resistance, elastomer-modified phenolic resins and oil-modified phenolic resins having excellent flexibility, and the like are examined and some of the resins are practically used. However, a modification method of improving the mechanical properties has not yet been found.
Here, as a method of satisfying the above requirement, it has been examined that a powdered inorganic material such as calcium carbonate, silica, alumina, or the like, which has excellent mechanical strength, is mixed into the phenolic resin. Particularly, a method in which a metal oxide is dispersed in the resin using the hydrolysis and polycondensation of metal alkoxide, which is referred to as sol-gel reaction, has been frequently examined (for example, refer to Japanese Unexamined Patent Publication No. H11-92623). However, the method is not yet practically used due to problems in production, cost and the like.